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Title: Bifurcation physics of magnetic islands and stochasticity explored by heat pulse propagation studies in toroidal plasmas

Abstract

Bifurcation physics of the magnetic island was investigated using the heat pulse propagation technique produced by the modulation of electron cyclotron heating. There are two types of bifurcation phenomena observed in LHD and DIII-D. One is a bifurcation of the magnetic topology between nested and stochastic fields. The nested state is characterized by the bi-directional (inward and outward) propagation of the heat pulse with slow propagation speed. The stochastic state is characterized by the fast propagation of the heat pulse with electron temperature flattening. The other bifurcation is between magnetic island with larger thermal diffusivity and that with smaller thermal diffusivity. The damping of toroidal flow is observed at the O-point of the magnetic island both in helical plasmas and in tokamak plasmas during a mode locking phase with strong flow shears at the boundary of the magnetic island. Associated with the stochastization of the magnetic field, the abrupt damping of toroidal flow is observed in LHD. The toroidal flow shear shows a linear decay, while the ion temperature gradient shows an exponential decay. Lastly, this observation suggests that this flow damping is due to the change in the non-diffusive term of momentum transport.

Authors:
 [1];  [2];  [2];  [1];  [1];  [3];  [1];  [2];  [4];  [2]
  1. National Inst. for Fusion Science, Toki (Japan); Graduate Univ. for Advanced Studies, Toki (Japan)
  2. National Inst. for Fusion Science, Toki (Japan)
  3. General Atomics, San Diego, CA (United States)
  4. Kyushu Univ., Kasuga (Japan). Research Inst. for Applied Mechanics
Publication Date:
Research Org.:
General Atomics, San Diego, CA (United States)
Sponsoring Org.:
USDOE; National Institute for Fusion Science (NIFS-NINS)
OSTI Identifier:
1375943
Grant/Contract Number:
FC02-04ER54698; FG03-97ER54415; AC05-00OR22725
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Nuclear Fusion
Additional Journal Information:
Journal Volume: 56; Journal Issue: 9; Journal ID: ISSN 0029-5515
Publisher:
IOP Science
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; stochasticity; magnetic island; bifurcation; heat pulse propagation

Citation Formats

Ida, K., Kobayashi, T., Yoshinuma, M., Suzuki, Y., Narushima, Y., Evans, T. E., Ohdachi, S., Tsuchiya, H., Inagaki, S., and Itoh, K.. Bifurcation physics of magnetic islands and stochasticity explored by heat pulse propagation studies in toroidal plasmas. United States: N. p., 2016. Web. doi:10.1088/0029-5515/56/9/092001.
Ida, K., Kobayashi, T., Yoshinuma, M., Suzuki, Y., Narushima, Y., Evans, T. E., Ohdachi, S., Tsuchiya, H., Inagaki, S., & Itoh, K.. Bifurcation physics of magnetic islands and stochasticity explored by heat pulse propagation studies in toroidal plasmas. United States. doi:10.1088/0029-5515/56/9/092001.
Ida, K., Kobayashi, T., Yoshinuma, M., Suzuki, Y., Narushima, Y., Evans, T. E., Ohdachi, S., Tsuchiya, H., Inagaki, S., and Itoh, K.. Fri . "Bifurcation physics of magnetic islands and stochasticity explored by heat pulse propagation studies in toroidal plasmas". United States. doi:10.1088/0029-5515/56/9/092001. https://www.osti.gov/servlets/purl/1375943.
@article{osti_1375943,
title = {Bifurcation physics of magnetic islands and stochasticity explored by heat pulse propagation studies in toroidal plasmas},
author = {Ida, K. and Kobayashi, T. and Yoshinuma, M. and Suzuki, Y. and Narushima, Y. and Evans, T. E. and Ohdachi, S. and Tsuchiya, H. and Inagaki, S. and Itoh, K.},
abstractNote = {Bifurcation physics of the magnetic island was investigated using the heat pulse propagation technique produced by the modulation of electron cyclotron heating. There are two types of bifurcation phenomena observed in LHD and DIII-D. One is a bifurcation of the magnetic topology between nested and stochastic fields. The nested state is characterized by the bi-directional (inward and outward) propagation of the heat pulse with slow propagation speed. The stochastic state is characterized by the fast propagation of the heat pulse with electron temperature flattening. The other bifurcation is between magnetic island with larger thermal diffusivity and that with smaller thermal diffusivity. The damping of toroidal flow is observed at the O-point of the magnetic island both in helical plasmas and in tokamak plasmas during a mode locking phase with strong flow shears at the boundary of the magnetic island. Associated with the stochastization of the magnetic field, the abrupt damping of toroidal flow is observed in LHD. The toroidal flow shear shows a linear decay, while the ion temperature gradient shows an exponential decay. Lastly, this observation suggests that this flow damping is due to the change in the non-diffusive term of momentum transport.},
doi = {10.1088/0029-5515/56/9/092001},
journal = {Nuclear Fusion},
number = 9,
volume = 56,
place = {United States},
year = {Fri Jul 29 00:00:00 EDT 2016},
month = {Fri Jul 29 00:00:00 EDT 2016}
}

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Cited by: 5works
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  • The impact of sheared toroidal rotation on the evolution of pressure driven magnetic islands in tokamak plasmas is investigated using a resistive magnetohydrodynamics model augmented by a neoclassical Ohm's law. Particular attention is paid to the asymptotic matching data as the Mercier indices are altered in the presence of sheared flow. Analysis of the nonlinear island Grad-Shafranov equation shows that sheared flows tend to amplify the stabilizing pressure/curvature contribution to pressure driven islands in toroidal tokamaks relative to the island bootstrap current contribution. As such, sheared toroidal rotation tends to reduce saturated magnetic island widths.
  • This is a study of the effect of an energetic ion population injected from a neutral beam source on the nonlinear stability of magnetic islands. A kinetic theory is used to obtain the island current, which includes previously known modifications resulting from bootstrap currents and resistive interchanges. The island current thus obtained is used in an asymptotic treatment of the perturbed Ampere's law, which yields an evolution equation for the island width. It is shown that the island width saturates at a very small amplitude in tokamaks (with {Delta}{prime}{gt}0) when the energetic ion density profile peaks just outside the rationalmore » surface. Energetic ions can also be used to modify the island width in three-dimensional equilibria. It is found that in stellarator equilibria with large vacuum islands, energetic ions can be injected to reduce the widths of equilibrium islands.« less
  • Magnetic islands play an important role in determining the stability and confinement properties of toroidal plasmas. The nonlinear growth of magnetic islands can lead to major disruptions in tokamaks. In stellarators, which do not have a continuous symmetry, the formation of magnetic islands can set equilibrium beta limits. In the first part of this paper, a novel method is proposed for reducing drastically the size of {ital m}=2, {ital n}=1 islands in tokamaks by introducing a beamlet of energetic ions just outside the {ital q}=2 surface via parallel neutral-beam injection. A simple physical picture is given of the nonlinear stabilitymore » of a tearing mode in the Rutherford regime in the presence of energetic ions. This physical picture is supported by a kinetic calculation in the long mean-free-path regime in which the effect of bootstrap currents and resistive interchanges are retained. Estimates show that the energy of the beamlet required for the suppression is a small fraction of that used for Ohmic or beam-heating of the background plasma. In the second part of the paper, an equilibrium beta limit is obtained for stellarators by calculating the island widths induced by finite plasma pressure; the widths are then constrained by the criterion of island overlap. The theory is applied to the Heliotron-E device (Nakamura {ital et} {ital al}., Phys. Fluids B {bold 2}, 2528 (1990)). It is shown that some aspects of the experimental observations on internal disruptions in Heliotron-E can be interpreted as signatures of the equilibrium beta limit.« less
  • An analysis is presented of the dynamics of a helical magnetic island chain embedded in a toroidal plasma, in the presence of an externally imposed, rotating, magnetic perturbation of the same helicity. Calculations are carried out in the large aspect-ratio, zero-{beta}, resistive magnetohydrodynamical limit, and incorporate a realistic treatment of plasma viscosity. There are three regimes of operation, depending on the modulation frequency (i.e., the difference in rotation frequency between the island chain and the external perturbation). For slowly modulated islands, the perturbed velocity profile extends across the whole plasma. For strongly modulated islands, the perturbed velocity profile is localizedmore » around the island chain, but remains much wider than the chain. Finally, for very strongly modulated islands, the perturbed velocity profile collapses to a boundary layer on the island separatrix, plus a residual profile which extends a few island widths beyond the separatrix. Analytic expressions are obtained for the perturbed velocity profile, the island equation of motion, and the island width evolution equation in each of these three regimes. The ion polarization correction to the island width evolution equation, which has previously been reported to be stabilizing, is found to be destabilizing in all three regimes.« less